The present invention relates generally to plastic bottle formation and, more particularly, to a bottle drying apparatus for high speed production line drying of a liquid coating material applied to the interior surfaces of a plurality of plastic bottles.
Containers constructed from thermoplastic material have become widely used in the beverage industry, competing with traditional metal and glass containers. Attractive features of plastic containers include high resistance to rupture, transparency and light weight. A drawback of thermoplastic beverage bottles has been the relatively high cost of the plastic materials which are presently used to produce such bottles. Polystyrene, which costs significantly less than most plastics presently used to form bottles, is readily available as a by-product from the formation of more expensive plastic materials. However, there are presently no polystyrene containers in commercial use as containers for pressurized beverages because of a number of production related problems. Polystyrene bottles to be economically feasible must be capable of being produced rapidly and, as a finished product must be able to contain carbonated beverages under pressure without significant loss of carbonation from the beverage or without diffusion of oxygen from the atmosphere into the beverage.
The molecular structure of polystyrene is such that for wall thicknesses that are sufficiently thin to be economically feasible, a coating layer of another material must be applied over the inner surface of the bottle to limit gas diffusion to acceptable levels. Additionally, it has been found that a coating layer of chemically inert material may be necessary to isolate the bottle contents from physical contact with the polystyrene to prevent chemical reaction therewith and deterioration of the taste of the bottled beverage. The most efficient manner presently known by applicant to provide a barrier between the bottled contents and the polystyrene material forming the bottle is to apply first a coating layer of latex primer material and, after the primer layer has dried, to apply a top coat layer of polyvinylidene chloride latex (PVDC latex). A high speed spray coating apparatus for applying such coating layers is disclosed in U.S. patent application, Ser. No. 685,022 of Frank Leroy Shriver and Roger Alan Hahn, filed Dec. 21, 1984, which is hereby incorporated by reference for all that it discloses.
After a spray coating is applied to the interior surface of a plastic bottle, the coating layer must be dried before further production processes, including filling of the bottle with a beverage, may commence. The coating layer must be dried quickly for economic production of the bottles. However, if the bottles are exposed to extreme heat such as applied by a radiant heater or conventional convection driers, the plastic material from which the bottle is constructed may be adversely effected. Beverage cans are conventionally dried using convection heating methods during which high temperature air, e.g. 400 to 500 degrees Fahrenheit, is blown onto the inside and outside surfaces of the can. The extreme heat of this process and the difficulty of blowing air into the small opening of a bottle make this process impractical for plastic bottles. Additionally, even in can drying, this process is quite slow, taking on the order of two minutes to complete. If a plastic bottle is exposed to extreme temperatures for even a short period of time, the strength of the bottle may be reduced. When exposed to extreme temperatures for a longer period of time, a bottle may physically deform. Thus, in order to provide an economically feasible production process for plastic bottles used in the beverage industry, it is necessary to dry a spray coating layer applied to the interior of a bottle in a rapid manner without subjecting the bottle to extreme temperatures.
Applicant has discovered that the PVDC coating layer applied to the interior of the plastic bottles has a tendency to form a surface film which inhibits further drying of the coating layer. In order to provide proper drying of this type coating material it is desirable to cause the portion of the coating layer adjacent the bottle wall surface to dry before the exposed surface of the coating layer begins to form a film. Thus, the bottle drying method should facilitate drying of the coating laye from the bottle surface outwardly toward the exposed surface.